The invention relates to an apparatus for generating and transporting a charged particle beam. It relates, in particular, to an apparatus for generating and transporting the beam of an electron linear accelerator (LINAC) used in radiotherapy.
A typical LINAC uses a magnet system to deflect (by 270.degree.) an electron beam toward an isocenter. The deflected beam is then transformed and shaped into a treatment beam having desired dimensional and energy characteristics.
On entering the magnet system, the beam contains electrons having a range of energies and trajectories. Optimally, these electrons should be deflected so that they exit the magnet system in a tight parallel beam which is centered around a central axis. To this end, a number of multi-magnet systems with highly sophisticated field configurations have been developed. These systems work, as disclosed for instance in U.S. Pat. No. 3,867,635, with energy selection filters. Such a filter is normally located in the plane of symmetry of the magnet system, because it is at that location where the radial dispersion of the various electron trajectories is most pronounced and is a monotone function of the energy dispersion. The filter contains a pair of beam shaping vanes, each radially displaced from the central electron orbit by predetermined amounts. By cutting off radial edges of the beam, the vanes limit the width of the energy band of the transmitted beam electrons to perhaps .+-.5% on either side of a preset energy value E.sub.0.
This value results from a tradeoff: the narrower the energy band, the better the quality of the beam exiting the magnet system, but the higher the beam current necessary for generating a treatment beam of a given intensity. Additionally, the optimum band width depends also upon whether the treatment beam consists of electrons or gamma radiation, i.e. whether the LINAC operates in an "e mode" or a "y mode". In the e mode, the original electron beam, which is scattered in a foil after bending, should be as monoenergetic as possible, and should ideally have an energy width of less than E.sub.0 .+-.2%. In the y mode however, the electrons of the original beam may be energetically spread. This is because the x-rays produced by the electron beam in a target have an extremely broad energy spectrum which is fairly independent of the electron energies. Consequently, a y mode electron beam may have an energy width of at least E.sub.0 .+-.10%, and such a wide energy band is not only acceptable but even attractive: because of the heavy losses in the target, the electron beam must have a beam current which is perhaps 100 times the beam current in the e mode. This means that in the y mode power supply and shielding problems play a major role and could be reduced if less electrons were filtered out of the beam.
Accordingly, the energy selection filter disclosed in U.S. Pat. 3,867,635 requires (a) a high power electron source, (b) bulky shielding blocks and (c) extensive means for improving the treatment beam characteristics in the e mode.
It is therefore an object of this invention to provide an apparatus for generating and transporting a charged particle beam with an adjustable energy selection filter.
It is a more specific object of the invention to provide an apparatus for generating and transporting a charged particle beam with an energy selection filter which transmits beam particles having energies within a defined energy range that becomes broader with increasing current strength of the incident beam.
It is another object of the invention to provide an apparatus for generating and transporting a charged particle beam with a self-adjusting energy selection filter.
It is yet another object of this invention to provide an apparatus for generating and transporting a charged particle beam with a simple, robust and easily attachable energy selection filter.
It is still another object of this invention to improve on the existing systems for generating and transporting charged particle beams.